It is customary to filter the air provided to occupied spaces by heating, ventilating, and air conditioning (HVAC) equipment. One convenient and effective way to do this is to filter the air entering the return air intake opening in the plenum or duct leading to the furnace or air conditioner. A typical air circulation system of a house for example, has a fan which while operating constantly draws air present within the occupied space into the intake opening for reheating or reconditioning, and this air is caused to pass through a filter to remove particulate contamination. The filter may be a simple mechanical filter with a disposable or renewable element, or may be electronic. The following description involves mechanical filters which do nothing more than trap these particles on the upstream filter surface or within the filter through which the air passes.
It is helpful at this point to define terms that will be frequently used in the description to follow. The medium of an air filter is the actual material which performs the filtering function. The air filter element is the disposable unit including the medium, and which is installed in, and after filling with trapped particles is removed from a plenum, duct, or housing. The air filter unit or simply air filter, is the entire filter assembly including the element and the non-disposable structure in which the element is mounted.
In residential systems, the medium often comprises a nominally one inch (2.5 cm.) thick rectangular woven glass fiber mat configured as a box-type filter element. The length and width dimensions of these filter media vary with the particular installation, but are typically each between one and two feet (30.5 cm. to 61 cm.). A flexible cardboard edging having a U-shaped cross section encloses the edge of the mat's periphery to form the air filter element and gives the element a generalized box shape. The edging provides some stiffness for the element. Other types of elements use pleated filter paper as the medium, again having the same nominal 1 in. thickness and U-shaped edging. These filter elements are available in a variety of widths and lengths to conform with the dimensions of the opening in which the element is to be installed. This filter format will be referred to hereafter as a shallow filter element or shallow format filter.
For systems having return air filtration and using mechanical filtration, a shallow filter element is often placed in the return air intake opening. These openings have centrally or inwardly projecting sheet metal or plastic flanges around the entire periphery of the opening. The flanges' outer surfaces all lie in a common plane. The filter's edging is pressed against the flange's outer surface by force from a grille cover having an internal ridge which presses against the edging's outer surface creates a nearly air-tight seal between the outer flange surface and the inwardly facing edging surface. This air-tight seal forces almost all of the air entering the plenum to pass through the filter element medium.
As one would expect, different types of air filters have different levels of efficiency. "Efficiency" in this context refers to the percentage of the total number of particles in the air stream within a given size range which the filter element can trap. The efficiency of filters varies with different particle size ranges. For example, a high efficiency filter medium can trap a significant percentage of particles whose size is on the order of 0.3 micron, where a low efficiency medium traps relatively few of them. There is also the consideration of overall efficiency as opposed to filter medium efficiency. Overall efficiency takes into account the unavoidable air leakage around a filter element mounted in its housing. Leaking air is completely unfiltered. Its particle load pollutes the stream of filtered air, resulting in an overall efficiency lower than the medium efficiency.
But efficiency is not the only measure of medium quality. It is also important that a filter not create a large pressure drop in the air passing through it. A large pressure drop requires a more powerful fan to force the required air volume through it. And if the pressure drop is too great, the medium will deflect and perhaps even burst or tear as the load of trapped debris obstructs ever more pores within the medium. The amount of pressure drop presented by a particular medium depends largely on the number of pores or openings per unit area of the medium, on the average minimum cross section area of the pores, and of course on the total area of the medium through which the air flows. To a certain extent, pressure drop is also dependent on the medium thickness.
Obviously, as a filter element loads up with debris during use, its pressure drop increases. This leads into a further consideration for filters, that of carrying capacity and filter element life. "Carrying capacity" refers to the number of particles the filter element can trap per unit area projected to the air stream before clogging up to a point where the ability to remove particles is impaired and/or the pressure drop across the filter element becomes unacceptable. ("Dust-holding" capacity is an industry term which we intend to be substantially equivalent to carrying capacity.) Other things being equal, carrying capacity is directly related to total medium area. The capacity of mat filters which trap some of the particles within their volume may also depend to some extent on their thickness. Carrying capacity is one factor in determining the life of the element and thus the cost of filtering the air.
Advances in filter technology has led to improvements in each of these characteristics. Nevertheless, it is still true that there are tradeoffs between efficiency, pressure drop, and carrying capacity. For example, as a filter medium becomes more efficient, its pressure drop typically increases because the individual pores become smaller, other things being equal. Of course, it may be possible to add more pores per unit area, but the problem of adding pores is not trivial. Carrying capacity is closely related not only to the number of pores or area available for adsorbing particles, but also to pore size. As the average size of the pores decreases, efficiency usually increases, but the increased number of particles trapped per second and smaller pores cause the medium to more quickly clog, reducing its life.
An easy way to minimize pressure drop and maximize capacity is to increase total medium area. This fact has led to the development of pleated filters. These pleated filters are made from a long strip of filter paper medium which is folded back and forth on itself to form a series of pleats. So long as the adjacent pleat panels do not touch each other the air can easily flow through the individual panels. In order to maintain the topology of the pleats under the force created by the normal pressure drop across the medium, it is possible to insert combs on the downstream side of the medium which have individual teeth between each pair of adjacent pleat panels to prevent the pleats from collapsing against each other from the force created by the pressure drop across individual panels.
Mechanical filter elements typically now in use in HVAC systems lack the efficiency which some experts believe is needed for adequate quality of the filtered air. Environmental health studies and empirical experience both show that it is not only the larger particles which these shallow format mat and pleated filters do trap that affect air quality. Smaller particles such as tobacco smoke, mold spores, bacteria, pollen, etc. which pass through many filters without being trapped, can also cause allergy or health problems for some people. And of course, even small particles can accumulate to an extent which creates a film of dust on hard surfaces and causes fabrics on furniture and in window treatments to become dirty or discolored. Secondly, shallow format filters have a relatively small carrying capacity.
More recently, improved filter elements have been developed whose pressure drop and carrying capacity is superior to that of shallow format mat and pleated filters. These elements typically have relatively deep pleats (4-5 in. or 10-12.5 cm.) to provide a relatively large medium area providing the improved pressure drop and carrying capacity. These deep pleat elements are intended for use in return air ducts having chambers capable of receiving such filter elements. In a preferred design, the filter elements collapse into a relatively small volume for shipping. They have relatively rigid cardboard or plastic end strips or panels which detachably mate with reusable side panels to form a reasonably rigid rectangular filter element assembly. See U.S. patent application Ser. No. 08/738,276 by Osendorf et al. filed Oct. 26, 1996, which application was continued in application Ser. No. 08/967,115 filed Nov. 12, 1997 and issued on Nov. 24, 1998 as U.S. Pat. No. 5,840,094 for an example of such a collapsible filter element which can be assembled into a deep format pleated filter element using a pair of special side panels. The filter element assembly is inserted into the chamber, placing the filter element directly in the return air stream. The chamber's design seals the edges of the filter assembly reasonably well against peripheral leakage, improving the overall filtering efficiency. Such a deep pleat filter element and indeed, any type of filter element substantially thicker than shallow format filter elements and to which the invention described below is applicable, will be called a deep filter element or deep format filter.
It would be advantageous to replace shallow format return air intake opening-mounted filters with deep format pleated filters. However, the dimensional disparity between these deep format pleated filter elements and the shallow format filter element does not allow the former to directly replace the latter in a return air opening installation.